If you’re still wondering why Facebook ponied up a cool billion dollars for photo sharing app Instagram, Experian Hitwise might have just shed some light on it with some interest statistics.

According to Hitwise, Instagram became the 10th most visited social network in the UK last month. That’s absolutely massive, considering that most people see it as “just an app”. Of course, a lot of those photo views come on the web thanks to sharing functionality that blasts photos out to Twitter and Facebook.

In March 2012 alone, Instagram had almost 13 million UK visits on the web, an increase from 300,000 from the same time last year. It’s that type of growth that definitely got Facebook’s attention. That, and the fact that analysts were starting to consider the service a “social network”, which is the space that Facebook must own as it moves forward with going public.

Most of the usage for Instagram still comes from the US, as the service saw 3.8 million visitors in just one week last month.

Interestingly, Facebook is the number one source of traffic for Instagram photos, at least in the US:

Instagram upstream websites Instagram just became the 10th most visited social network in the UK

Was Instagram a real threat to Facebook overall? Probably not, but when it comes to photos, the app was definitely stealing marketshare quickly. Whenever you see something like Instagram pick up stream in multiple countries, you know that you have a hit on your hands.